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Topic Review (Newest First)

12-25-2008 11:31 AM

joe_padavano

Quote:

Originally Posted by Kevin45

Not being a smartazz, just curious. How would the rear "come around" if it locked up first? It seems to me if the front locked first, then the rear could come around. That is one of the reasons truckers were unhooking their front brakes, so the rear would pull them to a stop and not jacknife.

Traction is a dynamic situation. When the tire is rolling, the coefficient of friction in the direction of travel is lower than the coefficient of friction sideways. You're rolling in one direction and sliding in the other. This is what allows you to corner. If the tire is locked, you are now sliding in both directions and there is equal traction (or lack thereof) in both the forward and lateral directions. Thus, any minor off-axis force will cause the locked tires to move in a different direction.

12-25-2008 10:01 AM

KULTULZ

Quote:

Originally Posted by Kevin45

Not being a smartazz, just curious.

Not being a smartazz....

Quote:

How would the rear "come around" if it locked up first? It seems to me if the front locked first, then the rear could come around.

That is one of the reasons truckers were unhooking their front brakes, so the rear would pull them to a stop and not jacknife.

To save an argument, take a toy car and tape the front wheels from turning. Give it a push. Then tape the rear wheels to prevent their turning. Give it a push.

Any trucker that disabled front axle brakes was not only a moron but flagrant of DOT Regs.

If you have a toy tractor-trailer, repeat the test above. A sliding wheel has less rolling resistance than a turning wheel. Less friction on a wet pavement (or dry too actually) and the locked wheel will overcome the resistance of the turning wheel. One reason you cannot steer in a panic stop.

12-25-2008 06:58 AM

Kevin45

Quote:

Originally Posted by KULTULZ

I cannot imagine anyway that such a combination could be balanced. The rear discs would be simply too agressive, locking the rear wheels and causing the rear to come around, especially with a panic stop.

Maybe a very extreme ABS system, but why bother?

Not being a smartazz, just curious. How would the rear "come around" if it locked up first? It seems to me if the front locked first, then the rear could come around. That is one of the reasons truckers were unhooking their front brakes, so the rear would pull them to a stop and not jacknife.

12-24-2008 08:40 PM

matt167

Quote:

Originally Posted by joe_padavano

You make it work by selling the front drums and getting disks. Sorry, but front drums/rear disks is a REALLY bad idea. You can use a proportioning valve to really dial back the rear brake pressure, but why bother.

actully, that would not work.. would need to dial back the front preassure, because of the self energization, the brake force on the front would not be balanced with the rear 50/50 like a front drum setup would be with 4 wheel drums... but any way you look at it, it's not a good idea

12-24-2008 01:29 PM

kleen56

I just went through this with my 56 Truck. I have a disc conversion on the front and running disc on all four corners. I changed my master cylinder out because the other one 74 Monte Carlo did not work with the disc on the rear. I used the 67 Corvette master cylinder and it worked great. If the MC is mounted on the firewall, you won't need the 2 lb residual valves in the line. I did put a adjustable proportioning valve in the rear line, Wilwood makes a nice one. That way, you can adjust the rear brakes so they don't lock up, etc. Do a search in here, there are several postings on this subject. Good luck, Merry Christmas.
Ed

12-24-2008 12:37 PM

31chevy

When I do find the money to buy the front disc kit what do i need in the all disc system as for mastercylinder, and valves in between

12-24-2008 10:02 AM

kleen56

It's possible to do by using a disc /drum master cylinder and switching the lines. Basically the reverse of what is normal. I agree with everyone here, it's dangerous and I would not waste the time or effort to make it work. A majority of your stopping ability comes from the front brakes. Why would you want drum on the front and disc on the rear? Not a good option. I would wait and do it right... There is one thing I don't go cheapo on and that is brakes. I like to make sure I get home in one piece. And with that, I hope everyone has a safe holiday season.

12-24-2008 08:51 AM

NEW INTERIORS

They are ''NOT'' Design to work that way,Don't do it!!!!

12-24-2008 08:17 AM

timothale

4 wheel disc

My son put a corvette master Cyl in his 22 dodge. 4 wheel disc. He hasn't got to do any real testing yet.

12-24-2008 08:11 AM

oldred

DON'T DO IT! I saw this tried once on a mustang using a Granada rear with discs (yep you could get a Granada with rear discs) and try as he might he never could get it to work right. Until I saw Timothale's reply I could not understand why it would seem to work, sort of, at slow speeds but not at high speeds and high stopping loads. This kid had that thing over to my shop several times trying different valves and such but when it became obvious it was not going to work I convinced him to spend the money for the front discs. He did get it to the point where it was not really dangerous to drive but under heavy braking it just did not feel right, come to think of it we only tried it on a dry road so I am not sure how safe it would have been on wet pavement.

12-24-2008 07:48 AM

timothale

drum brakes

The newer drum brakes are self energizing. 48 and earlier fords were not. that's why in the old days we all wanted lincoln or '50's ford pickup setups..the force you apply at the pedal IS NOT a straight line graph of the stopping power at the wheels... Disc brakes come closer to a straight line. TRying to match that setup with self energizing fronts and disc rear. . to get good operation at slow. hard and panic stops would be very very dififcult. Front brakes do most of the stopping so front disc's-- rear drums is easier to get a compromise braking system . 4 wheel disc brakes with 2 master cylinders and an adjustment balancing bar like the original 427 cobra makes it easy to adjust front to rear balance... Several companies now make that style setup, Check your speedway catalog. for the wilwood setup. .. retired Ford engineer.

12-24-2008 07:38 AM

KULTULZ

I cannot imagine anyway that such a combination could be balanced. The rear discs would be simply too agressive, locking the rear wheels and causing the rear to come around, especially with a panic stop.

Maybe a very extreme ABS system, but why bother?

12-24-2008 06:42 AM

aosborn

It is all a question of balance. The brakes themselves don't care what axle they are on. While it is an odd combination like you wrote, it could be done. You would need a 10lb residual check valve in the drum brake line, and a 2lb in the disc brake line if the master cylinder is below the floor. An adjustable proportioning valve would also need to be installed in the line to the rear. You would need to do some slow speed testing to check your brake balance, and work up to higher speeds. I would suspect the fronts would lock up long before the rears even with the proportioning valve wide open, but there are too many variables to be sure.

Keep this thought in mind regarding your proposal...Just because you can, doesn't mean you should...

Merry Christmas,
Andy

12-23-2008 10:04 PM

31chevy

make sense thanks , I thought you could do it but i'll save my money

12-23-2008 07:11 PM

joe_padavano

Quote:

Originally Posted by 31chevy

I know that it is less than ideal to do this but how can i make it work. what metering valves, residual valves and such do i need to make this work. Disc brakes came with the rear end i bought and I have an opertunity to get a front drum setup up from a friend that will work.

You make it work by selling the front drums and getting disks. Sorry, but front drums/rear disks is a REALLY bad idea. You can use a proportioning valve to really dial back the rear brake pressure, but why bother.

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